1. Field of Invention
The present invention relates generally to reflective film to reflective polarizing films where the polarized reflectivity may be varied by impressing an electric field across the film. The pitch of the liquid Cholesteric order varies non-linearly throughout the thickness of the film to produce a very broad band reflective film. Such films may be used to great advantage in electro-optical glazing structures having reflection, semi-transparent, and transparent modes of operation which are electrically-switchable for use in dynamically controlling electromagnetic radiation flow in diverse applications.
2. Brief Description of the Literature
Broadband reflecting polarizers were introduced in application Ser. No. 08/550,022 (now U.S. Pat. No. 5,691,789) entitled xe2x80x9cSingle Layer Reflective Super Broadband Circular Polarizer and Method of Fabrication Thereforxe2x80x9d by Sadeg M. Faris and Le Li which was filed Oct. 30, 1995. Such broadband polarizers are made by producing a single layer having cholesteric liquid crystal order where the pitch of the liquid crystal order varies in a non linear fashion across the layer. A narrow band, switchable polarizing single layer reflector is disclosed in European patent application 0 643 121 A, published Mar. 15, 1995. A switchable polarizing single layer reflector having a broader bandwidth is disclosed in PCT application WO97/2358, published Jul. 3, 1997. General references on polymer dispersed liquid crystals may be found in detail in xe2x80x9cPolymer Dispersed Liquid crystal displaysxe2x80x9d, by J. W. Doane, a chapter in xe2x80x9cLiquid Crystalsxe2x80x9d, Ed. B. Bahadur, World Scientific Publishing, Singapore, and xe2x80x9cCLC/polymer dispersion for haze-free light shutters, by D. Yang et al. Appl. Phys. Lett. 60, 3102 (1992). Smart Window Design is treated in xe2x80x9cElectrochromism and smart window designxe2x80x9d, by C. Granqvist, Solid State Ionics 53-56 (1992) and xe2x80x9clarge scale electochromic devices for smart windows and absorbersxe2x80x9d, by T. Meisel and R. Baraun, SPIE 1728, 200 (1992). The above identified US patents and other references are hereby incorporated by reference.
It is an object of the invention to provide a single layer polarizing film having a very wide bandwidth which is switchable.
It is an object of the invention to provide a switchable reflecting polarizing filter having a very wide bandwidth which is controllable by an electric field.
It is an object of the invention to provide a switchable reflective film having little variation in the reflectivity outside of the reflective bandwidth of the film.
It is an object of the invention to provide a xe2x80x9csmart windowxe2x80x9d using a polarizing reflective film having a very wide bandwidth.
It is an object of the invention to provide a xe2x80x9csmart windowxe2x80x9d using a polarizing reflective film having a very wide bandwidth combined with a reflective multilayer polymer film having a very wide bandwidth.
It is an object of the invention to provide a xe2x80x9csmart windowxe2x80x9d using a polarizing reflective film having a very wide bandwidth combined with a reflective multilayer polymer film having little variation in the reflectivity outside of the reflective bandwidth of the film.
It is an object of the invention to provide a xe2x80x9csmart windowxe2x80x9d using a polarizing reflective multilayer polymer film having a very wide bandwidth combined with a light scattering layer for further control of transmitted light.
The present invention provides a single layer polarizing reflective film comprising a cross linked polymer matrix mixed with low molecular weight liquid crystal molecules. The liquid crystal molecules are oriented with respect to the surface of the film and to each other in cholesteric order, and the pitch of the cholesteric order varies non-linearly across the thickness of the film so that the film reflects circularly polarized light having a broad bandwidth. The ratio of the amount of liquid crystal molecules to the amount of cross-linked polymer is chosen so that the liquid crystal molecules may rotate reversibly in an electric field, and hence destroy the liquid crystalline order responsible for the reflectivity of the polarized light. There is sufficient high molecular weight cross linked polymer material to ensure that the film is not liquid, and to ensure that the low molecular weight material does not diffuse after manufacture of the film, and to ensure that the low molecular weight material returns to the cholesteric ordered state when the field is removed.